Suppression of cross talk



,Oct 6, 1942.- B. G. BJoRNsoN AEmu.V 2,298,190

SUPPRESS ION vOF' CROSS -TALK N 2 N e Q a I a v7 :s A:

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SUPPRESSION OF CROSS-TALK y Filed May s, 1941 2 sheets-sheet 2 GAIN GAINY K DEcREAseR INcREAsgR A A F/G. 2

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Y B CJo/PNSON A TTORNEY Patented Oct. 6, 1942 "UNITED STATES 'ATE N T OF F I CE SUPPRESSION OF CROSS TALK Application May 3, 1941, Serial No.391,660

V6 Claims.

The present invention relates to cross-talk reduction between telephonecircuits, particularly between the two channels of a twin-channel ortwin-single side-band radio system.

In this type of system, of itself known in the art, one telephone.message is sent as the upper side-band and another telephone message issent as the lower side-band of the same radio frequency wave. Such .aIdouble side-band wave is usually built up by a series of modulationsteps involving, in certain stages at least, use 4in common by bothmessages of modulating and power amplifying apparatus which is liable togive rise to intermodulation products appearing as crosstalk from onechannel into the other. n

During normal use of such a system there will be times when speech isbeing received by both channels sin'iultaneously and other times whenspeech is being received by one channel while the opposite receivingchannel is in condition to receive but is momentarily idle. A workingassumption is made that when both channels #are receiving speech, thereceived speech effectively masks the cross-talk so that no cross-talkreducing means need be used under s-uch condition.

It is a general object of the invention to reduce cross-talk into anidle channel which is in receiving condition.

In the detailed disclosures of this specification several ways ofrealizing this object in practice will be illustrated and described.This and the various subsidiary objects and features of the inventionwill be made more apparent in such detailed disclosures and will beparticularly pointed out and distinguished in the claims.

Since both terminals of a radio telephone system of the type to whichthe invention is particularly, though not exclusively, adapted willcomprise a radio transmitter and a radio receiver and may in practiceb'e Vduplicates of each other, it is deemed sufficient in the drawingsto 'show only one terminal in each illustrative embodiment, it beingunderstood that the opposite terminal may be the counterpart of the oneshown.

In each of the embodiments for reducing crcssv talk to be disclosedherein, it will be observed that when one channel lis idle andthe otheris carrying speech, the idle channel has its loss increased to suppressthe cross-talk currents therein. When speech is applied to such idlechannel the increase in loss is removed.

In the drawings:

Fig. l is a simplified schematic sketch of the essential lcir-cuitfeatures employed at one twoway terminal of a twin-channel radiotelephone if system in accordance with one form of the invention;

Fig. 2 is a similar circuit diagram showing a modification applied tothe receiving channels. In this ligure the transmitting channels are notillustrated;

Fig. 3 is a simpliiied schematic circuit diagram of one manner ofapplying cross-'talk suppression to a transmitter, according to theinvention; and

Fig. 4 shows amodied form of one `portion of the-'circuit of Fig. 3.

In each of the figures of the drawings the showing of the relays andcontrol circuits has been simplified. In practice various means knownand commonly employed in the art would be used to facilitate operation,provide the necessary timing and hang-over characteristics `andotherwise meet the technical requirements of the particular serviceconditions.

Fig. 1 shows land lines L1 and L2 connected to transmit out overrespective channels III and II leading to twin-channel radio transmitterI2 and to .receive incoming radio communications from twinchannel radioreceiver I3 over respective channels III and I5. The transmitting andreceiving channel branches are associated with the given line by meansof the customary hybrid coil I-I and balancing articial line N. Theradio transmitter and radio receiver may be of suitable knownconstruction such as is indicated in Taylor and Wright Patent 2,179,106,November 7, 1939. Each transmitting channel III, II and each receivingchannel III, I5 is shown as including a privacy circuit. This may be afrequency inver-ter or a split band privacy or other suitable type suchas are known in the art, and the arrangement may follow the moredetailed disclosure .of the Taylor and Wright patent.

Each channel branch includes adjacent the land line an amplifier I6,I'I, I8 or I9 (vcgad) whose gain is automatically adjusted to constantoutput volume by known means such as disclosed in Hogg-Doba Patent1,853,974, April 12, 1932, or in application of H. L. Barney, Serial No.370,600, filed December 18, 1940. These are useful where lines L1 and L2lead to a toll switching point where they may be connected to lines ofvarying length or where they may carry conversations of weak talkers atsome times andstrong talkers at other times.

Receiving channel I4 includes a noise reducer 20 having a controlcircuit 2|, and, similarly, receiving channel I5 includes a noisereducer 22 with control circuit 23. Each of these may suitably be of thetype shown in N. C. Norman Patent 2,063,334, December 8, 1936,especially Figs. 3 and '1. They are used here as in the patent toimprove the signal noise ratio by variably amplifying incoming waves insuch a way as to depress in the energy scale weak oscillations relativeto strong. This function is performed by means of the control circuitwhich develops from the incoming oscillations a rectified controlvoltage for determining the gain of a variable gain amplier in the box20 or 22 such as to provide a large loss to weak oscillations and noloss or even a gain for stronger incoming waves. In the presentdisclosure the noise reducers areused to suppress cross-talk, as willpresently be described.

If speech is incoming on channel I from radio receiver I3 it is likelyto be heard as cross-talk on line L1 provided that line is neithertransmitting nor receiving speech over its receiving y channel I4. Thecoupling producing the crosstalk may be anywhere in the system ahead ofthe noise reducer, but, as noted above, is most likely to be in thedistant transmitting station. If apparatus is arranged under theseconditions, however, to introduce a high loss into channel I4 at thispoint, the cross-talk will be effectively suppressed. The manner inwhich this result is accomplished may be seen from following through asequence of operations now to be described.

The incoming speech on channel I5 from radio receiver I3 after passingthe privacy circuit goes through noise reducer 22, amplifier I8 and intoline L2. Relay 24 is in receiving or normal condition with its backcontacts closed. Receiving AD (ampliiier-detector) 25 is energized bythe received speech and operates relays 26, 21 and 28. Relay 26 disablestransmitting AD-29. Relay disables the circuit branch leading fromAD-Bll of channel I4 to relay 3| preventing operation of the latter.Relay 28 conditions noise reducer 20 for operation by substituting fornormal bias source 32 the large negative battery 33 in series with thebias control resistor 49 of the control circuit 2| (corresponding toresistor 49 of the Norman patent) The effect of relay 28 in shifting itsarmature is to change circuit 20 from a zero loss circuit to aZO-decibel loss circuit for weak oscillations of cross-talk level.

If under these conditions, speech from the distant talker with Whom thesubscriber on line L1 is communicating should come in via radio receiverI3 and channel I4, some of this speech will pass into the controlcircuit 2I which will then adjust the circuit 20 to low loss permittingthe speech to pass through to line L1. Speech is now being received inboth channels I4 and I5 and it is assumed that the speech in eachchannel is strong enough to mask the cross-talk. When the receivedspeech in channel I4 operates AD-30 disabler relay 34 operates torestore the noise reducer 20 to its normal fixed bias condition byreleasing relay 28. If desired, control circuit 2I may be left tofunction during speech reception, relays 21 and 34 being optional.

Condenser 35 is for the purpose of allowing the loss to be introducedand removed gradually in noise reducer 20 and may be proportioned toprovide any desired time constant.

The system is entirely symmetrical so that noise reducer 22 serves inthe same way to suppress cross-talk in line L2 when channel I4 isreceiving and'channel I5 is in receiving condition but idle.

When a receiving vogad (voice operated gain adjusting device) is used ina multichannel system, cross-talk may cause false operation of the gainincreaser circuit of such vogad. To prevent this, a circuit as shown inFig. 2 may be used in accordance with one feature of this invention. Inthis gure only the receiving channels I4 and I5 are represented, thecorresponding transmitting channels being omitted for simplicity. Alsothe various switching relays for disabling or enabling respective sidesof the circuits have been omitted. Otherwise the system may beconsidered the same as in Fig. 1 but omitting the noise reducers andinserting the cross-controls now to be described.

The vario-amplifiers or vogads I6 and I8 have been shown more fully inFig. 2 by indicating in connection with each a gain increaser and a gaindecreaser, which, of course, are assumed present in Fig. 1 also. Aheadof gain increaser 40 of vogad I6 is a resistance pad 4I normallyshortcircuited by relay 42 in its unenergized condition. Similarlyassociated with vogad I8 are gain increaser 43, pad 44 and relay 45.

It is a property of the vogad that it does not raise its gain until theincoming waves exceed a certain minimum amplitude, so that it will notreadjust its gain in response to noise of lower volume level thanspeech. The sensitivity is set for a given noise condition so that theproper marginal operation is obtained. The sensitivity can be reduced byinserting loss in the gain increaser input and the sensitivity isincreased by removing this loss.

Referring to Fig. 2, if speech is being received in channel I5 fromradio terminal I3, cross-talk may be produced in channel I4 under theconditions outlined in the description of Fig. 1. Such cross-talk wouldhave a tendency to cause adjustment of the gain of the vario-amplifierIB, which would mean an increase in its gain since such cross-talk isweaker than normal speech and the vogad normally adjusts its gain to agiven level of output corresponding to normal or acceptable receivinglevel for normal speech. Such tendency toward false operation of thevogad is reduced, however, by operation of AD-41 and relay 42 whichinserts pad 4I in the gain increaser circuit of the vogad I6, therebydecreasing its sensitivity and preventing a change of its gain inresponse to the cross-talk.

Since the circuits are symmetrically arranged, an entirely similaraction takes place to prevent false operation of the gain increasercircuit of vogad I8 by cross-talk from channel I4 when channel I5 isidle, by operation of AD-46 and relay 45 to insert pad 44 in the gainincreaser circuit by vogad I8.

If desired, disablers (not shown) similar to relays 21 and 34 of Fig. 1may be added to this circuit each adapted to be energized in parallelwith a respective relay 42 or 45 to disable the energizing circuit ofthe opposite relay 45 or 42, respectively.

Several advantages (as discussed below) can be secured where it is foundfeasible to suppress the cross-talk .at the transmitter, such as infairly low frequency systems that permit of separation of thetransmitted side-bands by filtering, between the output of the final orpower amplifier and the antenna. Systems for cross-talk suppression atthe transmitter are illustrated in Figs. 3 and 4.

In Fig. 3 only the transmitting channels I0 and Il are shown, since thereceiving channels are not directly involved and may be supplied fromthe earlier figures. It is assumed that the wavelengths are such as topermit of separating channel No. 1 on the output side of radiotransmitter l2 from channel No. 2 by the band-pass filters 53 and 5|inserted between the output side of the transmitter and the radiatingantenna 52. Speech in either channel interrupts the other (in theabsence of any speech input to the latter) by disabling thecorresponding filter. Assuming that the cross-talk products occur in themodulating or amplifying circuits of the transmitter such cross-talkproducts from an active channel into an idle channel are prevented frombeing radiated or are at least reduced to harmless level.

For instance if speech is present in the input of channel No, l but nospeech is being applied to the input of channel No. 2, A13-53 operatesrelays 54, 55 and 55, the latter interrupting the circuit through filterI of channel No. 2, so that cross-talk from channel No. 1 is nottransmitted. If speech comes on to the input of channel No. 2, AD-Eloperates relays 58 and 59, the latter restoring the circuit throughfilter 5l. Relays 54 and 53 are part of the regular vodas equipment. TheXs in the output leads from ADs 53 and 5l indicate disabling pointsoperated from the receiving channels (not shown).

When speech is present in channel No. 2 but channel No. 1 is idle, relayEl) is operated from CII What is claimed is:

l. In a 'twin-channel radio telephone system in which respectiveside-bands of the radiated wave carry different speech messages and inwhich cross -talk from an active channel to an idle channel occurs,means operated under control of speech-representing waves in an activechannel for introducing loss into the opposite channel when idleto'reduce the magnitude of the cross-talk currents therein, and meansoperated in waves impressed on the latter channel for removing saidloss.

AD-5'l (along with relays 58 and 59) disabling channel No. 1 at theinput to filter 50.

In Fig. 4 the elements shown may be substituted for those to the rightof broken line Y--Y in Fig. 3. Elements 5G and 5l are band suppressionlters or they may be tuned circuits, two-terminal or four-terminal, suchas ordinary anti-resonant or rejector `circuits to give a requisitedegree of suppression of frequencies most effective in producingcross-talk. To secure beneficial results a high degree of suppressionmay not be necessary in some cases, but a 10-decibel discrimination mayvbe very helpful. simple character can be used.

Relays and B are operated in the same manner as in Fig. 3 and wheneither relay is operated it interrupts a normal short circuit around the-corresponding band suppression circuit thereby in effect inserting thesuppression circuit in the corresponding channel.

A suppression of the cross-talk at the transmitting end would have manyadvantages, the following being the most important, (1) the operation ofthe cross-talk suppressor would be more positive at the transmitting endthan at the receiving end, where the signal amplitudes are influenced byfading, etc.; (2) the ratio between cross-talk amplitude and signalamplitude for a given signal intensity would be approximately constantat the transmitting end, whereas at the receiving end fading introducesvariability; (3) cross-talk suppression at the transmitting end is notsubject to trouble from locked-out speech; (4) in one operation all thereceiving devices, vodas, receiving vogads, etc., would be protectedfrom being unduly influenced by crosstalk; and (5) eificient cross-talksuppression, where possible at the transmitting end, might allow moredistortion in the ampliiers.

The invention is not to be construed as limited to the particularcircuit arrangements that have been disclosed as illustrative examples,but the scope is dened in the claims, which follow.

Thus, circuits of rather f 2. A system according to claim l in whichsaid channels are receiving channels, said first means comprising avariable loss circuit normally having low loss to received Waves and aconditioning circuit operative to increase the loss under control ofreceived speech in the opposite channel, and said second meanscomprising a control circuit for decreasing the loss in response toreceived'waves only in excess of cross-talk level.

3. A system according to claim 1 in which said channels are transmittingchannels, said first means comprising frequency selective loss deviceseach adapted to introduce loss in the frequency band of one respectivechannel but not the other and a circuit controlled from an activechannel for effectively inserting a said device into the oppositechannel when idle, and said second means comprising switching meanscontrolled by speech-representing energy in the input of the latterchannel for disabling said circuit.

4. in a multichannel telephone system cornprising physically adjacentchannels adapted for transmitting different speech messages in the samedirection, a variable loss device in each channel, such device having acontrol circuit for automatically adjusting the loss of the device undercontrol of waves in the respective channel and dependent upon themagnitude of such waves, and means controlled by speech waves in4 eitherchannel for decreasing the sensitivity of the control circuit of theloss device of the opposite channel.

5. In a telephone system comprising physically adjacent channels adaptedto transmit separate speech messages in the same direction and tocross-talk into each other, a vario-gain amplifier in each channelhaving a control circuit normally set to zero loss in the absence ofspeech in the opposite channel, said control circuit operating to insertgain in the device in response to input waves of greater than cross-talkmagnitude, and

means controlled from the opposite channel for inserting loss-in thedevice in the absence of input. waves of greater than cross-talkmagnitude. 6. In a twin-channel, single side-band radio telephone systememploying a noise reducer in the receiving circuit of each channel at aterminal of the system, means for reducing the effects of interchannelcross-talk comprising a separate detector controlled by the input to thereceiving noise reducer of the respective channel at said terminal,responsive to incoming telephone signals to condition the receivingnoise reducer in the other channel for operation.

BJORN G. BJORNSON. HAROLD J. FISHER.

response to speech-representing'

